144MHz Radio Beacon Circuit


The transmitter on 2 meters 144 MHz was designed mainly for use by radio amateurs as radio beacon and to this end, it produces a signal of good quality and without unwanted harmonics. T1 transistor, in association with cristall X1, works as oscillator on 36 MHz. L1-C3 filter eliminates any tendency of the circuit oscillation on 12MHz (fundamental frequency of the crystal).

L2-C4 circuit is tuned on the fourth harmonic of oscillator signal (144 MHz). This signal reaches the antenna after passing through a buffer consisting of T2, a FET with 2 gates. The signal modulator (amplitude) is applied to the second gate of the buffer.

It maintain a reduced power output of the transmitter, about 10-40 mW. Modulator signal is generated by N1, a oscillator which, thru T3, switches the transmitter in on status or repose, the switching freq is between 0.1 Hz and 0.5 Hz. The digital signal from the gate of T2 configures the modulator signal. Gate N2 generates a rectangular signal with a frequency of 0.1-1 Hz. So long as the exit of N is N3, N4 oscillates on a freq about 1 kHz. So, to the T2 gate is a periodically burst signal, with frequency of 1 kHz, and this signal is used for transmitter modulation.
By adjusting trimmers capacitors C4, C7 and C8, the transmitter will be calibrated for maximum power output. L2 and L3 coils are made of copper enameled with Ø = 0.8 mm: L2 = 5 turns, the outlet from the first turn toward ground; L3a = 3 turns and L3b = 2 turns. L3a and L3b coupling must be adjusted until power is obtained maximum output power.

The circuit consumes 20 mA, which allows the transmitter to work with a battery powered PP3, 9V, several hours.

Radio Beacon circuit schematic

Radio beacon 144 MHz


Sorry, comments are closed!